Various forms of variable dampers have been proposed for use in wheel suspension systems for the purposes of improving the ride quality and achieving a favorable motion stability of the vehicle. In a common conventional variable damper, a rotary valve is incorporated in the piston for varying an effective area of an orifice that communicates the two chambers on either side of the piston with each other, and such a rotary valve is typically actuated mechanically by using a suitable actuator. More recently, it has become more common to use magneto-rheological fluid (MRF) for the actuating fluid of the damper, and control the viscosity of the fluid by supplying corresponding electric current to a magnetized liquid valve (MLV) which is incorporated in the piston. According to such an arrangement, the overall structure can be simplified, and the response property of the damper can be improved. See U.S. Pat. No. 6,260,675, Japanese patent laid open publication No. 2006-069527 and Japanese patent laid open publication No. 2006-273223, for instance. The contents of these prior art references are hereby incorporated in this application by reference.
In a typical control process for an MRF variable damper, a damper control device determines a target damping force for each wheel according to the lateral acceleration and fore-and-aft acceleration of the vehicle body, and determines the target value of the drive electric current (target electric current) that is to be supplied to the MLV according to the target damping force and the stroke speed of the damper. When the vehicle is in motion, the outputs of the sensors for detecting lateral acceleration and fore-and-aft acceleration inevitably include low level noises, and such noises may cause the target damping force to be excessively great or small. In particular, when the vehicle is traveling straight at a constant speed, because the absolute values of the lateral acceleration and fore-and-aft acceleration are small, the influences of such small noises may become significant, and may prevent the ride quality and motion stability of the vehicle from being favorably controlled. Japanese patent laid open publication No. 2000-273223 discloses a method in which the changes in the damping force are reduced when the stroke speed of the damper is near zero by reducing the target current from the normal level.
This previous proposal however has the problem that the drive electric current changes excessively rapidly under certain conditions. For instance, when the vehicle is traveling at a high speed, each wheel moves vertically by a short stroke at a high frequency, and this causes a corresponding rapid contraction and extension of the damper at a high frequency. This in turn causes such rapid changes in the drive electric current for the MLV that the damper becomes unable to provide a stable damping force. This is due to the fact that a damper is not able to produce a positive damping force even though the target damping force may be positive if the stroke speed is negative. Under such a condition, the previously proposed system reduces the drive electric current to zero. Therefore, when the damper undergoes a small stroke movement at a high frequency, the level of the drive electric current for the MLV tends to change rapidly. This is demonstrated in the graph of FIG. 24. This not only prevents a stable damper control but also applies undesired stress to the drive circuit for the MLV which adversely affects the damper control device.
There is also a demand to control the damping force so as to simultaneously achieve various control modes, such as those of the skyhook control, roll control, pitch control and so on. If such plural control modes can be achieved without sacrificing any one of the control modes, it would be possible to further improve the behavior of a vehicle than was possible with any prior control mode.